Vinayak Vipradas
Ansys Employee

Hi Jared, 

Results for multiple frequency cases will use the number of cycles last defined S-N data point for undefined, low-stress values. Extrapolation of the S-N data curve outside your data points is not done. So at high frequencies, there will be many cycles per second so a shortened life could be predicted if your S-N data does not cover appropriate stress range. 

The infinite life setting is used to reduce the effect that small stress cycles have on the calculated fatigue damage. For non-constant amplitude loading, cycles with very small alternating stresses can over predict the fatigue damage. For example, with non-constant amplitude proportional loading, the history data may produce many cycles with near zero alternating stress. These near zero stress cycles should produce negligible fatigue damage. It is extremely conservative to assume that the life associated with these stress cycles is 1e6 cycles.

For more information about fatigue life results, please refer to the following resources:
Fatigue Results ( 
Frequency-Based Fatigue using Harmonic Analysis in Ansys Mechanical — Lesson 4 - YouTube

Vinayak Vipradas